Using the industry concordances:

Each concordance has four columns:
(orig ind code) -- Industry code vintage to be converted
naics12		-- Corresponding 2012 NAICS code
weight		-- Weight for this conversion to 2012 NAICS
source		-- Flag indicating the source for this conversion
		   (either from the official concordances, Fort-Klimek, or both)

Download the concordance corresponding to the correct CBP year.
    Years	|   Concordance file name
----------------+----------------------------
1977-1987	| sic77_to_naics12.csv
1988-1997	| sic87_to_naics12.csv
1998-2002	| naics97_to_naics12.csv
2003-2007	| naics02_to_naics12.csv
2008-2011	| naics07_to_naics12.csv

To get employment counts on a 2012 NAICS basis, merge the concordance to the data 
by the (orig ind code), multiply the original employment count by the weight to 
get the 2012 NAICS employment, and collapse by FIPS code and 2012 NAICS.
